1. Field of the Invention
The present invention relates to a liquid crystal aligning film and a liquid crystal display device. More specifically, it relates to a liquid crystal aligning film having a polybenzylimide skeleton and to a liquid display device prepared by using this film.
2. Description of the Prior Art
A twisted nematic (which will hereinafter be abbreviated as TN) mode is one in which the aligned direction or a nematic liquid crystal provided between a pair of upper and lower electrode substrates has been twisted 90.degree. on the surfaces of the substrates. It has been used extensively. There is another mode in which the aligned direction of a nematic liquid crystal is twisted from 180.degree. to 300.degree. and it is known as a super nematic mode (which will hereafter be abbreviated as STN). Furthermore, in recent years, in order to perform a matrix display or a color display by means of liquid crystals, many developments have been made regarding an active type twisted nematic mode using a majority of picture element electrodes and MIM (metal--insulating phase--metal) elements or TFT (field effect type thin film transistor) elements for their ON-OFF operation.
The common problem for all these modes are that when the same image is displayed for a long time and then a transfer to a new image is made, the previous image remains as an after image. In particular, the improvement in the after image is very important in order to obtain a high quality liquid crystal display device.
It is considered that the causes of the after image may be due to a potential difference, that is to say: since a DC component is added to a liquid crystal display device, electric double layers due to the impure ionic components contained in the liquid crystal are produced on the interfaces of the liquid crystal aligning film. The offsetting of electric charges is given between the upper and lower substrates and such a potential difference as to keep the condition stable is created. Since DC components cannot be removed, particularly from the TFT elements due to the characteristics of the elements, these problems of after image are easily highlighted more than those in TN and STN and look serious. Furthermore, a higher voltage holding ratio is required for preventing the flickering of the image in the TFT mode.
As for an aligning film used in such liquid crystal display devices, organic type films, such as polyimides and polyamides, are mainly used. For example, Japanese patent application laid-open No. Sho 51-65960 discloses a liquid crystal display device having a liquid crystal aligning film using a polyimide resin constituted of the repeated unit represented by: ##STR3##
There was, however, a problem in that an after image is easily caused in such a device using a polyimide aligning film obtained by using such a polyether compound.